The present invention relates generally to an apparatus and method for connecting two shafts. More particularly, the invention pertains to an apparatus and method for connecting two shafts which may be of dissimilar materials. The invention also pertains to radially restraining the lower end of a shaft assembly with a steady bearing.
It is known in various industrial mixing applications to have a vessel that contains liquid and/or solid material that is to be mixed. Typically, a shaft extends into the vessel, and has impellers radially projecting from, and/or radially supported by, the rotating shaft. The shaft is rotationally driven by a motor, often external to the vessel. As the shaft rotates, the impellers are moved through the material and effect a mixing of the material.
In some of the vessel impeller shaft arrangements that are used, it is known for a driven shaft to extend downwardly from the top of the vessel, and have its lower end terminate before reaching the bottom of the vessel, so that the lower end of the shaft is in a suspended state from the top. In some situations where the lower end of the impeller shaft is freely suspended, or xe2x80x9coverhungxe2x80x9d the lower end of the shaft will tend to deflect radially (i.e., undergo off-axis movement) due to the many lateral forces that are present at the impeller(s) on the mixer shaft. To inhibit this movement, it has been known to place a steady bearing arrangement mounted to on the bottom or sides of the mixing vessel, which has a bushing that surrounds the impeller shaft near its lower end and provides axial constraints to the shaft. This arrangement also raises the lateral natural frequency of the shaft. In this manner taller vessels and longer shafts can be used.
Impeller shafts are sometimes constructed of one or more longitudinal sections. Because of the bending and torsional forces imposed on the impeller shaft, and because of the corrosive environments typically present in such mixer environments, it has been known to have the impeller shaft be constructed of one or more longitudinal sections of a metal or metal alloy such as stainless steel.
In the case of a stainless steel impeller shaft that is constrained by a lower steady bearing as described above, it has been found that the frictional contact area between the lower end of the impeller shaft and the bushing of the steady bearing can lead to undesirable wear.
A contributing factor to this undesirable wear is the fact that in a corrosive or oxidizing environment such as is often present in the mixer systems, stainless steel develops a thin oxidized coating on its surface. Normally, where friction is not present, as on the parts of the shaft not in contact with the bearing, this thin coating develops only to a certain thickness, thereafter, any further oxidation on the surface of the shaft is inhibited. However, in the region of the bushing contact, the bushing tends to continually wear off the oxidized coating as it develops. As a result, the bushing tends to continually wear away the surface of a stainless steel shaft where it contacts the shaft, reducing the overall life of the bearing/shaft connection.
Silicon carbide is not generally used for shaft purposes due to its extreme brittleness, high cost, and the ease with which it can be fractured. It does, however, possesses excellent corrosion resistant properties, extreme hardness, very high modulus of elasticity, very low coefficient of thermal expansion, etc. Metal shafts do not generally simultaneously posses these attributes.
Accordingly, it is desirable to have a shaft and bearing arrangement that can be subject to less wear than the above described arrangement. It will also be desirable to have such an apparatus and method that can be conveniently installed and serviced.
It is therefor a feature and advantage of the present invention to provide the connection of a stub shaft made of a wear resistant material, onto the lower end of the driven mixer shaft.
It is another feature and advantage of the present invention to provide an apparatus and method for connecting shafts to one another that are made of dissimilar materials.
It is another feature and advantage of the present invention to provide such an apparatus and method that can be conveniently installed and serviced.
The above and other features and advantages are achieved through the use of a novel apparatus and method for connecting shafts as herein disclosed. In accordance with one embodiment of the present invention, an apparatus is provided for restraining an end of a rotatable impeller shaft. The apparatus includes a support hub connected to the end of the impeller shaft for rotation therewith. A stub shaft is rigidly mounted to the support hub for rotation therewith. A steady bearing surrounds at least a portion of the stub shaft. In another aspect, the stub shaft may be made of a ceramic, for example, silicon carbide. In another aspect, the stub shaft may be press-fit into the support hub.
In still another aspect, the invention provides an apparatus for restraining an end of a rotatable impeller shaft. The apparatus includes a stub shaft. A supporting means supports the stub shaft, and is connected to the end of the impeller shaft for rotation therewith. Bearing means are provided for radially restraining at least a portion of the stub shaft.
In another aspect, a method is provided for restraining an end of a rotatable impeller shaft. The method includes mounting a stub shaft in a support hub, attaching the support hub to the lower end of the impeller shaft, and placing the stub shaft into a bearing assembly.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.